In a Micro-electromechanical Systems (MEMS) processing, it is often necessary to remove a dielectric layer from a surface of a silicon wafer, e.g. removing a passivation layer on the bottom of a back cavity of the silicon wafer. The passivation layer can be a silicon dioxide layer or silicon nitride layer. A conventional method of removing the passivation layer on the bottom of a back cavity of the silicon wafer is a wet etching, i.e., using an etching solution to corrode the silicon dioxide or silicon nitride to remove the passivation layer of the silicon wafer. However, since the dimension of the back cavity of the silicon wafer is usually in the nanometer or micrometer scale, the etching solution is difficult to reach the back cavity or infiltrate the surface of the passivation layer completely due to the surface tension of the liquid, such that the passivation layer is difficult to be corroded, which results in some residual of the passivation layer and a lower corrosion accuracy.